The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating?

A minor modification of the reported
procedure for the synthesis
of a corrole dimer that is fused by the cyclooctatetraene (COT) unit,
(H3tpfc)2COT, allowed for its isolation in 18%
yield. Of the two redox isomers that this interesting macrocycle does
form, the current focus is on the reduced form, in which each subunit
resembles that of monomeric corroles with a trianionic N4 coordination core. The corresponding bis-gallium­(III) complex was
prepared as an entry into the potentially rich coordination chemistry
of (H3tpfc)2COT. Both X-ray crystallography
and DFT calculations disclosed that the COT moiety is essentially
planar with very unusual nonalternating C–C bonds. The same
holds true for the bis-gallium­(III) complexes [(Ga-tpfc)2]­COT­(py)2 and [(Ga-tpfc)2]­COT­(py)4, obtained with one and two pyridine molecules coordinated to each
metal ion, respectively. The electronic spectra of both the free base
and the gallium­(III) complexes display an extremely low energy band
(λmax at 720–724 nm), which points toward
extensive π delocalization through the COT bridge. This aspect
was fully addressed by examining the interactions between the two
corrole subunits in terms of electrochemistry and DFT calculations
of the oxidized and reduced macrocycle. The new near-IR bands that
appear upon both oxidation (λmax 1250 nm) and reduction
(λmax 1780 nm) serve as additional supporting evidence
for this conclusion.